fix(twitch): derive emailVerified from email presence

Twitch's Helix GET /users response has no email_verified field, so the
provider previously hardcoded emailVerified: true. That's fine when an
email is returned (Twitch only surfaces verified addresses), but the
email field can be absent — producing a nonsensical emailVerified: true
alongside a null email.

Set emailVerified to !!userInfo.email so verified status tracks the
actual presence of a verified email.

apps/backend/src/oauth/providers/twitch.tsx

@@ -40,7 +40,7 @@ export class TwitchProvider extends OAuthBaseProvider {
       displayName: userInfo.display_name,
       email: userInfo.email,
       profileImageUrl: userInfo.profile_image_url,
-      emailVerified: true,
+      emailVerified: !!userInfo.email,
     });
   }